There is popularized such a printer that forms an electrostatic latent image by exposing a surface of a photosensitive drum in condition that, on the photosensitive drum, a plurality of positions (elements) are arranged along a line extending in the main scanning direction so as to be irradiated with an optical beam, develops the electrostatic latent image by using color material such as toner, and transfers onto a printing medium the color material image formed through the development. In relation to such a printer, one method has been proposed to improve the linearity of the output density, by arranging, on the photosensitive drum, the plurality of positions (elements) to be irradiated with an optical beam along a plurality of lines, each of which extends in the main scanning direction and which are arranged adjacent to one another in a subsidiary scanning direction.